18,152 research outputs found
Nernst and Seebeck effect in a graphene nanoribbon
The thermoelectric power, including the Nernst and Seebeck effects, in
graphene nanoribbon is studied. By using the non-equilibrium Green function
combining with the tight-binding Hamiltonian, the Nernst and Seebeck
coefficients are obtained. Due to the electron-hole symmetry, the Nernst
coefficient is an even function of the Fermi energy while the Seebeck
coefficient is an odd function regardless of the magnetic field. In the
presence of a strong magnetic field, the Nernst and Seebeck coefficients are
almost independent of the chirality and width of the nanoribbon, and they show
peaks when the Fermi energy crosses the Landau levels. The height of -th
(excluding ) peak is for the Nernst effect and is
for the Seebeck effect. For the zeroth peak, it is abnormal with height
for the Nernst effect and the peak disappears for the Seebeck effect.
When the magnetic field is turned off, however, the Nernst effect is absent and
only Seebeck effect exists. In this case, the Seebeck coefficient strongly
depends on the chirality of the nanoribbon. The peaks are equidistant for the
nanoribbons with zigzag edge but are irregularly distributed for the armchair
edge. In particular, for the insulating armchair ribbon, the Seebeck
coefficient can be very large near the Dirac point. When the magnetic field
varies from zero to large values, the differences among the Seebeck
coefficients for different chiral ribbons gradually vanish and the nonzero
value of Nernst coefficient appears first near the Dirac point then gradually
extents to the whole energy region.Comment: 8 pages, 7 figure
Improved position measurement of nano electromechanical systems using cross correlations
We consider position measurements using the cross-correlated output of two
tunnel junction position detectors. Using a fully quantum treatment, we
calculate the equation of motion for the density matrix of the coupled
detector-detector-mechanical oscillator system. After discussing the presence
of a bound on the peak-to-background ratio in a position measurement using a
single detector, we show how one can use detector cross correlations to
overcome this bound. We analyze two different possible experimental
realizations of the cross correlation measurement and show that in both cases
the maximum cross-correlated output is obtained when using twin detectors and
applying equal bias to each tunnel junction. Furthermore, we show how the
double-detector setup can be exploited to drastically reduce the added
displacement noise of the oscillator.Comment: 9 pages, 1 figure; v2: new Sec.
Calculation of the current noise spectrum in mesoscopic transport: an efficient quantum master equation approach
Based on our recent work on quantum transport [Li et al., Phys. Rev. B 71,
205304 (2005)], where the calculation of transport current by means of quantum
master equation was presented, in this paper we show how an efficient
calculation can be performed for the transport noise spectrum. Compared to the
longstanding classical rate equation or the recently proposed quantum
trajectory method, the approach presented in this paper combines their
respective advantages, i.e., it enables us to tackle both the many-body Coulomb
interactionand quantum coherence on equal footing and under a wide range of
setup circumstances. The practical performance and advantages are illustrated
by a number of examples, where besides the known results and new insights
obtained in a transparent manner, we find that this alternative approach is
much simpler than other well-known full quantum mechanical methods such as the
Landauer-B\"uttiker scattering matrix theory and the nonequilibrium Green's
function technique.Comment: 13 pages, 3 figures, submitted to PR
Thermoelectric and Magnetothermoelectric Transport Measurements of Graphene
The conductance and thermoelectric power (TEP) of graphene is simultaneously
measured using microfabricated heater and thermometer electrodes. The sign of
the TEP changes across the charge neutrality point as the majority carrier
density switches from electron to hole. The gate dependent conductance and TEP
exhibit a quantitative agreement with the semiclassical Mott relation. In the
quantum Hall regime at high magnetic field, quantized thermopower and Nernst
signals are observed and are also in agreement with the generalized Mott
relation, except for strong deviations near the charge neutrality point
Noise properties of two single electron transistors coupled by a nanomechanical resonator
We analyze the noise properties of two single electron transistors (SETs)
coupled via a shared voltage gate consisting of a nanomechanical resonator.
Working in the regime where the resonator can be treated as a classical system,
we find that the SETs act on the resonator like two independent heat baths. The
coupling to the resonator generates positive correlations in the currents
flowing through each of the SETs as well as between the two currents. In the
regime where the dynamics of the resonator is dominated by the back-action of
the SETs, these positive correlations can lead to parametrically large
enhancements of the low frequency current noise. These noise properties can be
understood in terms of the effects on the SET currents of fluctuations in the
state of a resonator in thermal equilibrium which persist for times of order
the resonator damping time.Comment: Accepted for publication in Phys. Rev.
Electrical transport through a single-electron transistor strongly coupled to an oscillator
We investigate electrical transport through a single-electron transistor
coupled to a nanomechanical oscillator. Using a combination of a
master-equation approach and a numerical Monte Carlo method, we calculate the
average current and the current noise in the strong-coupling regime, studying
deviations from previously derived analytic results valid in the limit of
weak-coupling. After generalizing the weak-coupling theory to enable the
calculation of higher cumulants of the current, we use our numerical approach
to study how the third cumulant is affected in the strong-coupling regime. In
this case, we find an interesting crossover between a weak-coupling transport
regime where the third cumulant heavily depends on the frequency of the
oscillator to one where it becomes practically independent of this parameter.
Finally, we study the spectrum of the transport noise and show that the two
peaks found in the weak-coupling limit merge on increasing the coupling
strength. Our calculation of the frequency-dependence of the noise also allows
to describe how transport-induced damping of the mechanical oscillations is
affected in the strong-coupling regime.Comment: 11 pages, 9 figure
Current noise of a quantum dot p-i-n junction in a photonic crystal
The shot-noise spectrum of a quantum dot p-i-n junction embedded inside a
three-dimensional photonic crystal is investigated. Radiative decay properties
of quantum dot excitons can be obtained from the observation of the current
noise. The characteristic of the photonic band gap is revealed in the current
noise with discontinuous behavior. Applications of such a device in
entanglement generation and emission of single photons are pointed out, and may
be achieved with current technologies.Comment: 4 pages, 3 figures, to appear in Phys. Rev. B (2005
Non-equilibrium Entanglement and Noise in Coupled Qubits
We study charge entanglement in two Coulomb-coupled double quantum dots in
thermal equilibrium and under stationary non-equilibrium transport conditions.
In the transport regime, the entanglement exhibits a clear switching threshold
and various limits due to suppression of tunneling by Quantum Zeno localisation
or by an interaction induced energy gap. We also calculate quantum noise
spectra and discuss the inter-dot current correlation as an indicator of the
entanglement in transport experiments.Comment: 4 pages, 4 figure
Using single quantum states as spin filters to study spin polarization in ferromagnets
By measuring electron tunneling between a ferromagnet and individual energy
levels in an aluminum quantum dot, we show how spin-resolved quantum states can
be used as filters to determine spin-dependent tunneling rates. We also observe
magnetic-field-dependent shifts in the magnet's electrochemical potential
relative to the dot's energy levels. The shifts vary between samples and are
generally smaller than expected from the magnet's spin-polarized density of
states. We suggest that they are affected by field-dependent charge
redistribution at the magnetic interface.Comment: 4 pages, 1 color figur
Phase-change chalcogenide glass metamaterial
Combining metamaterials with functional media brings a new dimension to their
performance. Here we demonstrate substantial resonance frequency tuning in a
photonic metamaterial hybridized with an electrically/optically switchable
chalcogenide glass. The transition between amorphous and crystalline forms
brings about a 10% shift in the near-infrared resonance wavelength of an
asymmetric split-ring array, providing transmission modulation functionality
with a contrast ratio of 4:1 in a device of sub-wavelength thickness.Comment: 3 pages, 3 figure
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